The present disclosure relates to an image forming apparatus for charging a surface of a photoconductor.
Conventionally, a photoconductor including a photoconductive layer having a thickness of ten to several tens of μm and forming a surface for carrying electrostatic latent images is used in an image forming apparatus such as a printer or a copier. In an image forming apparatus using such the photoconductor, a phenomenon called image deletion may occur. The image deletion is a phenomenon in which an image is blurred or the periphery of an image is bled.
The image deletion occurs due to a reduction of surface resistance of the surface of the photoconductor. Specifically, by discharge from a conductive member, discharge products such as nitrate ions and ammonium ions adhere to the surface of the photoconductor. If these discharge products absorb moisture in the air to be ionized, the surface resistance of the photoconductor decreases. Electrostatic latent images formed on the surface of the photoconductor having a reduced surface resistance flow to the periphery to induce a potential drop. In this way, boundaries of the electrostatic latent images become unclear, resulting in the image deletion.
Further, in recent years, a surface of a photoconductor of a corotron or scorotron type has been charged using a charging member such as a charging roller arranged in contact with or in proximity to the surface of the photoconductor to reduce an ozone generation amount instead of a charging member such as a corotron or a scorotron arranged out of contact with the surface of the photoconductor. Thus, in recent years, a surface of a photoconductor has received discharge from a closer position and has been more easily worn and deteriorated. As a result, discharge products more easily adhere and the image depletion more easily occurs.
Accordingly, in recent years, there has been proposed a method for accurately setting an inter-peak voltage value of an alternating-current voltage applied to a charging member to charge a photoconductor to a suitable voltage value regardless of the aged deterioration of the photoconductor, the charging member and the like and a variation of an air environment around the photoconductor such as a humidity.
Specifically, a quadratic curve representing a relationship between the inter-peak voltage value and a current value of a current flowing from the charging member to the photoconductor is first determined. Then, two different low-pressure side inter-peak voltage values supposed to be on a lower pressure side than a voltage value of an inflexion point appearing when an inter-peak voltage is boosted in this quadratic curve are obtained. Then, a straight line passing through current values when alternating-current voltages having the obtained two low-pressure side inter-peak voltage values are applied is derived. Then, a high-pressure side inter-peak voltage value supposed to be on a higher pressure side than the voltage value at the inflexion point in the quadratic curve is obtained. Then, a straight line passing through a current value when an alternating-current voltage having the high-pressure side inter-peak voltage value is applied and parallel to a coordinate axis representing the inter-peak voltage value is derived. Thereafter, an inter-peak voltage value corresponding to an intersection of the above two derived straight lines is set as a proper inter-peak voltage value.